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The early peoples of Southern Mexico decorated pottery and painted pictures on walls with a stable blue pigment which is not found elsewhere in the world. Investigation of this blue was started over 30 years ago, but still the true nature of the blue color principle is unknown. Since the blue cannot be destroyed by boiling nitric acid, it does not seem to be vegetable or organic in origin. It is quite unlike azurite or natural ultramarine or other blue minerals which were employed as sources of blue pigment by other ancient peoples. The main obstacle in the investigation is the extreme scarcity of research material. The only samples of the blue available for testing are thinly painted films on potsherds and on wall painting fragments where it is mixed with lime plaster and other impurities. Although attempts to procure lump specimens of the blue, even in gram quantitives, have failed, some progress has been made. It is now known that the inorganic base of the blue pigment is a clay mineral called attapulgite. Ordinary attapulgite is nearly colorless. We still do not know what makes the clay blue; whether it is a special kind of attapulgite or if the Maya produced it artifically. In this paper all the evidence accumulated to date is reviewed.

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